The early 1900s brought slow and steady growth to the Boulder County Fair as permanent buildings replaced the tents at Roosevelt Park one by one, with some costs being borne by the City of Longmont. In 1921 a record-breaking crowd of 2,500 attended the opening day. The Fair prospered.
The only break in the continuity of the Fair occurred in 1946 when it had to be canceled because of the polio outbreak. But eventually, with the continued success of the event, more room and better facilities were needed. In 1976, the County Commissioners purchased the Affolter Corner Farm, 130 acres of land at the northeast corner of Nelson and Hover Roads. In the fall of 1978, the Fair opened in its present location in the first building at the new complex, which would be completed over the next 15 years.